Rust-preventive pigments used as a steel structure coating paint, a mirror back-coating paint, etc. must be free from lead and chromium, according to the RoHS directive (Directive on the Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment) and the WEEE directive (Directives on Waste Electrical and Electronic Equipment, directed to collection of equipment and recycle rate), which were put into effect in the European Union in 2006. Specifically, electrical and electronic equipment containing any of six designated hazardous substances (lead, mercury, cadmium, hexavalent chromium, polybromobiphenyl, and poly(bromodiphenyl ether)) in an amount greater than an allowed level cannot be sold. Among the designated hazardous substances, limitations are imposed on the allowable levels of hexavalent chromium and lead to 1,000 ppm or lower.
In Japan, the Ministry of Economy, Trade and Industry has expressed that “it is appropriate for any company to disclose information about environmentally hazardous substances; i.e., the six substances regulated by the RoHS directive, contained in their products.” Thus, such an action on the RoHS directive must be also taken in Japan, and other commercial products will be targeted in addition to electric and electronic equipment. Also, in Japan, the “Act on Confirmation, etc. of Release Amounts of Specific Chemical Substances in the Environment and Promotion of Improvements to the Management Thereof” (the regulation of Pollutant Release and Transfer Register (abbreviated as PRTR)) was enacted in 1999, and 354 chemical substances were designated. Some chromium compounds and lead compounds are designated substances.
In Japan, according to the JIS standards, rust-preventive coatings containing a rust-preventive pigment such as a chromium compound or a lead compound (e.g., red lead, basic lead chromate, zinc chromate, basic zinc potassium chromate, lead cyanamide, calcium plumbate, and basic lead sulfate) have been invalid since fiscal year 2014. Therefore, there is keen demand for an inexpensive, Pb-free, and Cr-free rust-preventive pigment exhibiting excellent anti-corrosive property.
Examples of currently employed Pb-free and Cr-free rust-preventive pigments include zinc salts such as zinc phosphate, zinc phosphite, and zinc molybdate; aluminum salts such as aluminum phosphate and aluminum molybdate; calcium salts such as calcium phosphate and calcium molybdate; and cyanamide compounds such as zinc cyanamide and zinc calcium cyanamide.
Other than the above zinc salts, a zinc salt cyanuric acid is disclosed as a component of a metal-surface anti-corrosive coating containing conventional ingredients and an organic zinc salt. A zinc cyanurate production method is also disclosed. In the method, zinc oxide is reacted with cyanuric acid in boiling water advantageously in the presence of a small amount of acetic acid serving as a catalyst, whereby a zinc cyanurate of interest is formed (see Patent Document 1).
Another production method is disclosed. In the method, zinc oxide and cyanuric acid are mixed together with water in a possible small amount of 10 to 80 mass % (with respect to paste), so as to prepare a kneadable paste, and shear force is applied to the prepared paste while the mixture is heated at 50 to 250° C., to thereby produce basic zinc cyanurate (see Patent Document 2). More specifically, in the Examples of Patent Document 2, desalted water, zinc oxide, and cyanuric acid are mixed by means of a paddle dryer equipped with a blade mill, to thereby prepare a paste having a water content of 70 mass %. The paste is heated at 105 to 110° C. and then dried under reduced pressure, whereby basic zinc cyanurate is yielded. The reaction is performed in a paste form, requiring post drying.
In still another disclosed production method, at least one selected from zinc oxide and basic zinc oxide is mixed with cyanuric acid and water such that the cyanuric acid concentration with respect to water is adjusted to 0.1 to 10.0 mass %, to thereby prepare a slurry. The slurry is wet-dispersed at 5 to 55° C. by the mediation of a dispersion medium, to thereby produce basic zinc cyanurate (see Patent Document 3).
In these basic zinc cyanurate production methods, reaction is performed in a slurry or paste state containing water and basic zinc cyanurate particles. Accordingly, for employing the thus-produced basic zinc cyanurate as a rust-preventive pigment, the obtained slurry or paste must be dried and pulverized under dry conditions to yield a micropowder. However, such a slurry or paste contains a large amount of water, and drying cost increases. Also, the dried product assumes a lumpy form with very high hardness. Thus, forming micropowder requires a very large amount of energy, increasing pulverization cost, which is disadvantageous.
Meanwhile, a rust-preventive coating paint or a corrosion-inhibiting coating agent for iron and the like employing zinc cyanurate as a rust-preventive pigment is disclosed. The disclosed undercoating paint for baking finish is based on an epoxy resin ester and is formed of a varnish (1373.5 parts by mass) and zinc cyanurate (Zn: 31.7%) (26.5 parts by mass) serving as a corrosion-inhibitor, wherein the varnish has the following composition: epoxy resin-castor oil fatty acid ester (fatty acid content: 42%, 60% in xylol) (400 parts by mass), zinc phosphate (110 parts by mass), microtalc (120 parts by mass), titanium dioxide (rutile) (80 parts by mass), barite (193.5 parts by mass), ethylene glycol (15 parts by mass), n-butanol (15 parts by mass), tetralin (30 parts by mass), a higher aromatic compound (110 parts by mass), a non-plasticized urea resin (65% in butanol) (110 parts by mass), and xylol (200 parts by mass) (see Patent Document 1).